Determining the position and direction of a conductive pipe (metallic casing, for example) accurately and efficiently is required in a variety of downhole applications. Perhaps the most important of these applications is the case of a blown out well in which the target well must be intersected very precisely by a relief well in order to stop the blowout. Other important applications include drilling of a well parallel to an existing well in Steam Assisted Gravity Drainage (“SAGD”) systems, avoiding collisions with other wells in a crowded oil field where wells are drilled in close proximity to each other and tracking an underground drilling path using a current injected metallic pipe over the ground as a reference. In SAGD is applications, a common practice is to use wireline systems for electromagnetic ranging between the wells. However, this requires access to both wells which is both time-consuming, and expensive. An alternative practice is to use electromagnetic logging-while-drilling systems, as these only require access to a single well.
However, the aforementioned approaches may only measure and process magnetic fields using inductive sensors. While this has served as a practical solution in the past, this could limit the operation to low frequencies and may not utilize all available electromagnetic information. Recently, other methods related to magnetic field gradient measurements have been disclosed, but these latter methods require the emplacement of multiple, proximal inductive sensors to approximate the magnetic field gradients, rather than measure the magnetic field gradients directly.
Accordingly, there is a need in the art for improved and/or alternative downhole ranging techniques.